Disposable diapers have met with increased commercial acceptance in recent years primarily because of their convenience, as opposed to cloth diapers, which need to be laundered once soiled. Many different constructions have been proposed and used, and some have met with widespread commercial success in spite of certain inadequacies in functional properties.
One of the most serious prior art problems has been the inability to provide a suitable construction that would keep moisture away from the surface of the diaper which comes into contact with the infant's skin and thereby avoid skin irritation and infection. Commonly assigned Mesek et al. U.S. Pat. No. 3,612,055 discloses several diaper constructions that function extremely well in keeping moisture away from an infant's skin, while at the same time handling a full volume discharge of urine.
These functions are accomplished by a multilayer diaper comprising, in order, a fibrous facing layer which is to be brought into contact with the infant's skin, a layer of highly porous, loosely compacted cellulosic batt, a paper-like, densified, highly compacted cellulosic fibrous layer integral with the loosely compacted batt and an impervious backing sheet adhered to the densified layer throughout the interface therebetween. The facing layer is of porous construction and its fibers have less wettability for water than the fibers of the loosely compacted batt, resulting in a tendency for liquid to flow from the facing web into the batt. The densified fibrous layer has a smaller average pore size than the loosely compacted batt, resulting in a tendency for liquid to flow preferentially from the batt into the underlying densified layer rather than to other areas of the batt, thus tending to restrict wetting in the batt to an area of moderate size. Liquid flowing into the densified layer tends to spread laterally because of its wicking action and liquid which might pass through the densified layer during discharge (when flow is rapid) is held back by the impervious backing sheet for sufficient time to permit absorption to take place. Liquid in excess of the absorptive capacity of the densified layer is forced back by the impervious layer into the dry portion of the loosely compacted batt, thus utilizing the additional absorptive capacity therein.
The facing layer in the above-described diaper is comprised of a mixture of long and short fibers that are held together by a binder having a wetting agent therein which reduces the water repellency of the facing layer, so that urine may readily pass therethrough and into the loosely compacted batt. The facing layer, as described in the aforementioned patent is made by air laying a web of mixed long and short fibers, impregnating the web with a binder emulsion containing a wetting agent, and then drying the impregnated web.
It is desirable that the facing layer have a low density and high loft while retaining its strength but it has not been possible by the method described in U.S. Pat. No. 3,612,055 to produce facing layers having densities less than about 0.05 g./cc. It is also desirable, as a matter of minimizing diaper cost, to have a facing layer of low fabric weight, such as a weight of about 11/2 oz./sq.yd.
Shepherd U.S. Pat. No. 3,759,775 describes a "puffed," or expanded web in which the fibers define strata, which, in turn, define fiber chambers larger than the expected interstitial spaces, which fiber chambers separate the fiber strata and act effectively as pores within the plane of the fabric. The fiber strata have a fiber density at or near that of conventionally laid dry fibers and the fiber chambers have a considerably lesser density.
The method described in U.S. Pat. No. 3,759,775 for making the expanded web involves impregnating a web with a vaporizable liquid which contains a binder in solution or emulsion and then rapidly vaporizing the volatile liquid to cause puffing of the web by entrapment of the generated vapors while setting the web in puffed condition by solidification of the binder.
A facing layer of high loft and low density can be made by the method of U.S. Pat. No. 3,759,775 from an air laid web of mixed long and short fibers. However, because the method of said patent requires entrapment of generated vapors within the web, there is a minimum web weight below which effective puffing is not achieved under practical conditions. It is not practical to puff a web weighing less than about 2 oz./sq.yd.; and facing layers at this weight are more costly than is desired in a competitively priced, disposable product.